Kofron et al U.S. Pat. No. 4,439,520 was the first to demonstrate a variety of photographic advantages to be realizable utilizing high aspect ratio tabular grain silver halide emulsions. The term "tabular grain emulsion" is applied to silver halide emulsions in which tabular grains, those having two parallel major faces larger than any remaining grain faces, account for greater than 50 percent of total grain projected area. The term "high aspect ratio" as applied to tabular grain emulsions indicates those emulsions in which the ratio of mean grain equivalent circular diameter (ECD) to mean grain thickness (t) is greater than 8. Kofron et al recognized the importance of controlling tabular grain thickness, with tabular grain thicknesses for most photographic applications taught to be less than 0.3 .mu.m and preferably less than 0.2 .mu.m.
Solberg et al U.S. Pat. No. 4,433,048, an improvement on the high aspect ratio tabular grain emulsions of Kofron et al, demonstrates that in silver iodobromide tabular grains a higher iodide concentration in a laterally displaced region of the tabular grain than in a central region results in higher photographic speeds without affecting granularity. That is, improved speed-granularity relationships are obtained. Solberg et al teaches either gradually or abruptly altering iodide concentrations during tabular grain precipitation. Solberg et al teaches that when the iodide level within the grain is gradually increased the central region of the grain need not be large, ranging from 2 to 50 mole percent, preferably 4 to 15 mole percent, of total silver forming the tabular grain. On the other hand, Solberg et al teaches to delay abrupt iodide concentration increases until the central region accounts for from 75 to 97 percent of the total silver forming the tabular grain structure. This delay in abrupt iodide addition is intended to avoid any disruption of the desired tabular form of the grains.
Although iodide nonuniformity accounts for improvement in speed-granularity relationships whether iodide concentrations are increased gradually or increased abruptly, the latter offers larger photographic advantages. Analytical investigations indicate that abrupt iodide concentration increases produce a distinct structure as compared to iodide that is gradually increased. One technique for observing this is by observing low temperature photoluminescence. When iodide is uniformly distributed within silver iodobromide tabular grains or non-uniformly distributed, but with gradual variations in iodide concentrations, exposure of the tabular grains at a temperature of 6.degree. K. to electromagnetic radiation at a wavelength of 325 nm results in a fluorescent emission having a peak intensity in the wavelength range of from 490 to 560 nm, but at 600 nm the intensity of the emission is less than 1.0 percent of the peak emission. When, however, iodide has been introduced into the grain structure abruptly, the luminescence stimulated in the same way is shifted so that at 600 nm the intensity of the emission is greater than 5 percent of the peak emission intensity. Chang et al U.S. Pat. No. 5,314,793 illustrates the advantages of photographic elements that employ high aspect ratio tabular grain silver iodobromide emulsions producedby abrupt iodide introductions.
Although it was recognized by Kofron et al that thin (&lt;0.2 .mu.m) high aspect ratio tabular grain emulsions are photographically preferred, not until recently has it become apparent that there are significant photographic advantages to be gained by employing high aspect tabular grain iodobromide emulsions that are ultrathin, where "ultrathin tabular grain emulsions" are understood to be those than exhibit mean tabular grain thicknesses of less than 0.07 .mu.m. Ultrathin tabular grain emulsions, particularly when the tabular grains account for a high percentage of total grain projected area are particularly useful in producing images of increased sharpness. Antoniades et al U.S. Pat. No. 5,250,403 demonstrates highly advantageous photographic elements employing these emulsions.
A significant advantage of ultrathin tabular grain emulsions is that they do not exhibit reflection maxima within the visible spectrum, as is recognized to be characteristic of tabular grains having thicknesses in the 0.18 to 0.08 .mu.m range, as taught by Buhr et al, Research Disclosure, Vol. 253, Item 25330, May 1985. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. In multilayer photographic elements overlying emulsion layers with mean tabular grain thicknesses in the 0.18 to 0.08 .mu.m range require care in selection to avoid reducing the imaging response of underlying emulsion layers by reason for reflecting light that these emulsions require for imaging. The choice of ultrathin tabular grain emulsions in building multilayer photographic elements eliminates spectral reflectance dictated choices of different mean grain thicknesses in the various emulsion layers overlying other emulsion layers. Hence, the use of ultrathin tabular grain emulsions not only allows improvements in photographic performance, it also offers the advantage of simplifying the construction of multilayer photographic elements.